A semiconductor chip comprising a silicon crystal grown on a substrate has been utilized as a solar battery. The construction of this solar battery is generally such that a connection lead is joined to a silicon crystal wafer in its predetermined region and electricity is output through the connection lead. In general, a solder plating is provided on the surface of the connection lead, for connection to the wafer.
A tin-lead-alloy-base solder is a classical material which has been actually used in other electrical components and has also been used to constitute the solder plating. In recent years, however, due to a fear that lead adversely affects the environment, studies have been made on the substitution of other lead-free solder, for plating, for this tin-lead-alloy-base solder.
The tin-lead-base solder, which has hitherto been extensively used, is a highly useful plating material characterized, for example, by excellent wettability, high bond strength, or good handleability while ensuring, for example, the electrical conductivity or mechanical strength possessed by the lead. Therefore, solder plating materials alternative to the tin-lead-base solder should be satisfactory in these various properties.
For example, tin-silver-base, tin-bismuth-base, and tin-copper-base solders have hitherto been regarded as promising lead-free plating material alternative to the tin-lead-base solder, and are expected to be utilized as a connection element in a large number of electrical components including materials for constituting the plating for joining a connection lead in the above-described solar battery.
FIG. 6 is a schematic diagram showing an example of the construction of a solar battery. In this drawing, numeral 1 designates a silicon wafer which receives sunlight and outputs electricity, and numeral 2 a silver-plated portion provided on the silicon wafer 1 in its narrowest possible restricted area in a predetermined region for high power output. A connection lead 3 is connected so as to be in the region of the silver-plated portion 2.
FIG. 7A shows the construction of the connection lead 3. The connection lead 3 comprises a copper strip 4 and a solder plating 5 provided on both sides of the copper strip 4. For example, this copper strip 4 has a thickness t1 of about 0.125 mm and a width W1 of about 1.5 mm, and on the other hand, the plating 5 has a thickness t2 of about 20 to 30 μm.
In the above construction, the connection lead, wherein the plating 5 formed of, for example, the above-described tin-silver-base, tin-bismuth-base, or tin-copper-base system has been provided in the joint, has a high level of properties as a lead using the solder free from lead, i.e., the so-called “lead-free solder,” and has been properly appreciated.
According to the conventional lead-free solder, however, an oxide layer is likely to be formed on the surface of the solder during production or use of the solder. In particular, heating for the connection of the connection lead 3 to the silver-plated portion 2 results in quick oxidation of the solder plating 5 which disadvantageously makes it difficult to provide predetermined bond strength.
This is a problem inherent in the lead-free solder and caused due to the fact that, since the solder is free from lead, the liquidus line is shifted to a higher temperature side which necessitates increasing the heating temperature. However, the problem of unsatisfactory bond strength is attributable to the material, as well as to the construction of the connection lead 3.
This will be explained in conjunction with FIG. 7B. FIG. 7B shows the state of connection of the connection lead 3 to the silver-plated portion 2. The plating 5 provided on the copper strip 4 is heat-melted and pressed against the silver-plated portion 2, whereby the connection lead 3 is joined to the silver-plated portion 2. In this case, an oxide layer (not shown) formed on the solder plating 5 upon heating is highly likely not to be broken by the pressing, and, consequently, adversely affects the state of connection. This is also a great factor in the unsatisfactory bond strength.
The problem of the unsatisfactory bond strength in its turn adversely affects other properties. Specifically, in the conventional connection lead for use in solar batteries, the plating 5 is generally designed to be thick so that the lack of bond strength is compensated for by the amount of the solder. Increasing the amount of solder, however, means that the solder flows out to a portion outside the predetermined region. For this reason, as indicated by A in FIG. 7B, the silicon wafer 1 is partially covered with solder flowed out from the silver-plated portion 2. This poses a problem of lowered power output efficiency of the solar battery.
Another problem involved in the structure of the connection lead 3 is the inclusion of gas bubbles in the solder joint. The construction of the lead 3, wherein the oxide layer formed on the solder plating 5 cannot be broken at the time of heat connection, leads to the inclusion of air due to the lack of fluidity of the plating 5. The inclusion of gas bubbles is causative of an increase in connection resistance which is in turn causative of a lowering in output efficiency of generated electric power.
Among the above-described problems, the formation of the oxide layer on the surface is not a problem involved in only solar batteries and is also a serious problem in other electrical components.
More specifically, the formation of the oxide layer is also evidently unfavorable in electrical components provided with a lead-free solder as a connection element, for example, printed boards having a lead-free soldered portion formed by flow or reflow, ball grid array-type printed boards having a plurality of lead-free solder balls arranged as terminals, single wires, twisted wires, and served shield wires, for electric wires, having a lead-free solder plating formed on the surface thereof, and a coaxial cable for medical or personal computer applications, comprising: an internal conductor of an extrafine alloy wire in a single wire or twisted wire form; an external conductor, of an extrafine alloy wire, provided on the internal conductor through an insulator; and a lead-free solder plating provided on the surface of the internal conductor and the surface of the external conductor, and this is causative of deteriorated product stability and reliability.